A Novel Modulation Technique Algorithms on Channel Coding and decoding

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Available online: http://edupediapublications.org/journals/index.php/IJR/ P a g e | 1285

A Novel Modulation Technique Algorithms on Channel

Coding and decoding

Nimmala Narendar

1

&Mr.K.Baskar

2

1

M-Tech Dept. ECE, SwarnaBharathi College of Engineering, Khammam

2

HOD&Associative professorDept. ECE, SwarnaBharathi College of Engineering, Khammam

Abstract

Orthogonal Frequency Division Multiplexing (OFDM) hasgained increased interest due to its robustness

against multipathinterference and high spectrum efficiency. OFDM is asuitable candidate for high data

rate transmission with forwarderror correction (FEC) methods over wireless channels. In thispaper, the

system throughput of a working OFDM system hasbeen enhanced by channel coding technique like

Reed Solomoncode& Turbo Code. Forward Error Correcting codes are a newclass of codes that can

achieve exceptional error performanceand energy efficiency at low signal-to-noise ratio. The simulation

is made with the development of Models in theSIMULINK & computer program written in MATLAB

sourcecode on the random data under additive white Gaussian noise(AWGN) channel. The simulation

results of estimated Bit errorrate (BER) show that the implementation of RS code with ½ -rated under

QPSK modulation technique is highly effective tocombat inherent interference in the communication

system.Fordecoding of Turbo code, MAP decoding algorithm is used. Incase of Turbo code BER

performance is substantially improvedby increasing number of iterations used in the decoding process.

Keywords:Channel Coding, Channel, decoding Algorithms, Modulation Technique etc.

1. Introduction

Orthogonal Frequency Division Multiplexing

(OFDM) is a Multi-Carrier Modulation

technique in which a single high rate

stream is divided into multiple low rate

data-streams and is modulated using sub-carriers

which are orthogonal to each other. Some of the

main advantages of OFDM are its multi-path

delay spread tolerance and efficient spectral

usage by allowing overlapping in the frequency

domain. OFDM is symbol based, and can be

thought of as a large number of low bit rate

carriers transmitting in parallel. All these

carriers transmitted using synchronized time and

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This is to ensure that the orthogonal nature of

the structure is maintained.

However, in the present study, an effort has

been made merely to concatenate the various

channel encoding codes to improve the reliable

reception performance of an OFDM wireless

communication system under different digital

modulation schemes such as QPSK, 32-QAM,

64-QAM. In almost all applications of

multi-carrier modulation, satisfactory performance

cannot be achieved without the addition of some

form of channel coding. In wireless systems

subjected to fading, extremely high

signal-to-noise ratios are required to achieve reasonable

error probability. In addition, interference from

other wireless channels is frequently severe. If

channel coding is applied, the performance of

OFDM is expected to be significantly improved

through time diversity of channel coding as well

as through inherent frequency diversity of the

OFDM [1] [2].

2. Related Work

Orthogonal frequency division multiplexing

(OFDM) is nowadays widely used for achieving

high data rates as well as combating multipath

fading in wireless communications. In this

multi-carrier modulation scheme data is

transmitted by dividing a single wideband

stream into several smaller or narrowband

parallel bit streams.

Each narrowband stream is modulated onto an

individual carrier. The narrowband channels are

orthogonal vis-à-vis each other, and are

transmitted simultaneously. In doing so, the

symbol duration is increased proportionately,

which reduces the effects of inter-symbol

interference (ISI) induced by multipath

Rayleigh-faded environments. The spectra of

the subcarriers overlap each other, making

OFDM more spectral efficient as opposed to

conventional multicarrier communication

schemes.

A. OFDM message

The OFDM message is generated in the

complex baseband. Each symbol is modulated

onto the corresponding subcarrier using variants

of phase shift keying (PSK) or different forms

of quadrature amplitude modulation (QAM).

The data symbols are converted from serial to

parallel before data transmission. The frequency

spacing between adjacent subcarriers is Nπ2,

where N is the number of subcarriers. This can

be achieved by using the inverse discrete

Fourier transform (IDFT), easily implemented

as the inverse fast Fourier transform (IFFT)

operation. As a result, the OFDM symbol

generated for an N-subcarrier system translates

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At the receiver, the OFDM message goes

through the exact opposite operation in the

discrete Fourier transform (DFT) to take the

corrupted symbols from a time domain form

into the frequency domain. In practice, the

baseband OFDM receiver performs the fast

Fourier transform (FFT) of the receive message

to recover the information that was originally

sent[2].

Figure 1 Basic OFDM System Architecture.

B. Interference

In a multipath environment, different versions

of the transmitted symbol reach the receiver at

different times. This is due to the fact that

different propagation paths exist between

transmitter and receiver. As a result, the time

dispersion stretches a particular received symbol

into the one following it. This symbol overlap is

called intersymbol interference, or ISI. It also is

a major factor in timing offset. One other form

of interference is intermarried interference or

ICI. In OFDM, successful demodulation

depends on maintaining orthogonality between

the carriers. We demodulate a specific

subcarrier N at its spectral peak, meaning that

all the other carriers must have a corresponding

zero spectra at the nth center frequency

(frequency domain perspective). Frequency

offsets lead to this criterion not being met. This

condition can seriously hinder the performance

of our OFDM system. Graph 3.1 below shows

that when the decision is not taken at the correct

center frequency (i.e. peak) of carrier

considered, adjacent carriers factor in the

decision making, thus reducing the performance

of the system [2].

Figure 2 Effect of Frequency Offset

(Maintaining Orthogonality).

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Figure 3. Cyclic Prefix.

OFDM demodulation must be synchronized

both in the time domain as well as in the

frequency domain. Engineers have found a way

to ensure that goal by adding a guard time in the

form of a cyclic prefix (CP) to each OFDM

symbol. The CP consists in duplicates of the end

samples of the OFDM message relocated at the

beginning of the OFDM symbol. This increase

the length T sym of the transmit message without

altering its frequency spectrum.

Where T data is the duration of one data symbol,

and N the number of carriers. The receiver is set

to demodulate over a complete OFDM symbol

period, which maintains orthogonality. As long

as the CP is longer than the channel delay

spread, τ max, the system will not suffer from ISI.

The CP is to be added after the FFT operation at

the transmitter and removed prior to

demodulation. The figure below who’s the

deteriotiation in performance when the CP is

closely matched by the delay spread. The signal

constellation is less tightly grouped, no doubt a

sign of less than accurate decoding.

3. Implementation

OFDM with forward error correction methods is

most suitable scheme to transmit information

efficiently BER using the Convolution code in

the presence of the fading channel is shown is

explained briefly in the review paper [8]. A type

of Convolution code, called turbo codes that

enable the reduction of errors in noisy channels

without the need to increase the signal power. A

turbo code consists three distinct part namely

encoder, interleave and decoder .The

performance of turbo code depend on the design

& implementation of all the three part. A typical

turbo encoder uses parallel concatenated

convolution codes (PCCC) in which data bits

are coded by two or more recursive systematic

convolution (RSC) coders separated by an

interleave. Turbo code uses two same block of

decoders, the decisions from one component

decoder are passed as input to another decoder

and this process is iteratively done for several

times to get more reliable decisions. The high

bit error correction power of turbo code

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and iterative decoding using extrinsic

information at the decoder [8] Turbo coded

AOFDM (TC AOFDM) system combines the

good features of AOFDM with that of turbo

code. Using the iterative property of turbo

codes, a large coding gain is achieved with

respect to an un-coded system.

3.1 TURBO ENCODER:

A Turbo encoder is built using parallel

concatenation of two recursive systematic

Convolution code separated by an interleave. A

typical turbo encoder is shown in figure 1. The

binary input data sequence is represented by dk=

(d1, d2…dN)The input sequence is passed into

the input of the first RSC coder, which

generates Yk-1. For the second RSC, the data

sequence is interleaved using random interleave

in which the bits are output in a pseudo-random

manner. The interleaved data sequence is passed

to a second RSC encoder, generate bit

streamYk2The output code sequence of the turbo

encoder is a multiplexed (and possibly

punctured) stream consisting of systematic code

bitsXk.along with the parity bits of first and

second encoders, Yk1 and Yk2.

Figure 4: Turbo Decoder.

4. Experimental Work

Fig 5: An ideal 16 QAM scatter plot (left)

impaired by a phase offset (middle) and a

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Fig 6: Scatter plot of a QPSK signal that shows

the signal before and after equalization, as well

as the ideal signal constellation.

Fig 7: Received signal scatter plot (left), after

frequency recovery (middle), and after phase

recovery (right).

5. Conclusion

To conclude, this major project gives the detail

knowledge of a current key issue in the field of

communications named Orthogonal Frequency

Division Multiplexing (OFDM). We focused

our attention on turbo codes and their

implementation. We described the encoder

architecture. In our case, the code is the result of

the parallel concatenation of two identical

RSCs. The code can be punctured in order to

fulfill bit rate requirements. The decoder

succeeded in its duty thanks to the decoding

algorithms that it is built around. We focused

mainly on the study of the MAP.

6. References

[1] M. K. Gupta, vishwas Sharma “To improve

BER of turbo coded OFDM channel over noisy

channel” in Journal of Theoretical and Applied

[2] Lou I Ilunga, Research Work on “Adaptive,

[3] “Block Turbo Code And Its Application to

OFDM For Wireless Local Area Network” PhD

thesis submitted by Hrudananda Prada.

[4] Liu Na Shi Wenxiao Wu Jiang “A Model of

Turbo Code Based on OFDM-CDMA” in IEEE

journal of 2006.

[5] B.Balaji Naik “Performance Of Turbo

Coded OFDM In Wireless Application” in

partial fulfillment of the requirements for the

award of Master of Technology degree in

Electronics and Communication Engineering

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Embedded system” during session 2007-2008 at

National Institute Of Technology, Rourkela

(Deemed University).

[6] Md. Dulal Haque1 , Shaikh Enayet Ullah2 ,

and Md.Razu Ahmed3 “Performance evaluation

of a wireless Orthogonal Frequency Division

Multiplexing system under various concatenated

FEC channel-coding schemes” in Proceeding of

11th International conference on Computer and

Information Technology (ICCIT 2008) 25-27

December, 2008, Khulna Bangladesh.

[7] Arun Agarwal, S. K. Patra, Senior Member

IEEE “Performance prediction of OFDM based

Digital Audio Broadcasting system using

Channel protection mechanisms” in IEEE

[8] Jun Zheng, “ Analysis Of Coded OFDM

System Over FrequencySelective Fading

Channels” Phd thesis Submitted to the Office of

Graduate Studies of Texas A&M University

[9] Yung-Chih Tsai, Yeong-Luh Ueng, “A

Tail-biting Turbo Coded OFDM System for PAPR

[10]D. Rajaveerappa, Abdelsalam Almarimi

“RSA/Shift Secured IFFT/FFT Based OFDM

Wireless System” in 2009 Fifth International

Conference on Information Assurance and

Security.

11]Hanjong Kim,” Performance improvement

of Block Turbo Coded OFDM System Using

channel state information” the 23rd international

conference on circuits/systems, computers and

communications (ITCCSCC 2008).

[12] J. Terry, and J. Deiskala, OFDM Wireless

LANs: A Theoretical and Practical Guide, Sams

Publishing, Indiana, 2002.

[13]Haixa Zhang, Feng Zhao, Dongfeng Yuan,

Mingyan Jiang, “Performance of turbo code an

WOFDM system on rayleigh fading channels,”

Proceedings, IEEE, vol. 2, pp.1570-1573, Sept

2003.

[14]J.W. Blakert, E.K. Hall, S.G. Wilson,

“Turbo code termination and interleaver

conditions,” Electronics Letters, Vol. 31, Issue

24, 1995, pp. 2082-2084.

Authors Profile

Name: NIMMALA NARENDAR

B-Tech in SwarnaBharathi College Of

Engineering, Khammam, percentage is 72.46% ,

year of completed April 2014.

M-Tech [ECE] (Electronics and Communication

Engineering)College:SwarnaBharathi College of

Engineering,Khammam.

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Name:Mr.K.BASKAR, M-Tech(P.hd)

He is working as Associative professor in

Department of Electronics&communication

Engineering at SWARNA BARATHI

COOLEGE OF ENGINEERING (SBCE),

KHAMMAM.He had 14 years Teaching

experience. His research interests is Digital